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Viable mice created from two female eggs

Scientists create viable mice by fusing two eggs, one of which has been …

A surprising number of animals can reproduce via a process called parthenogenesis, in which females produce viable and fertile offspring without getting a male involved. Although this is rarely the reproductive method of choice, even vertebrates such as sharks and lizards have reproduced without males when sexual reproduction wasn't an option in zoos and aquariums. Parthenogenesis doesn't seem to be an option in mammals, though, as both male and female mammals modify the genomes in their germ cells in ways that cause lasting changes in their expression, a process termed imprinting. But a research team has now figured out a way of avoiding these problems, and have created mice with two mothers, and no involvement with a father.

The researchers built this breakthrough on two pieces of knowledge. The first is that cells early on the path to oocyte (egg cell) development haven't been imprinted yet. The second is that researchers have identified a small number of areas in the genome that undergo opposite imprinting in the two sexes. The researchers generated mice carrying deletions of two of the imprinted areas, and then isolated early oocytes that had not started the imprinting process from these mice.

These cells could be fused with a normal oocyte, creating a cell with a normal diploid genome. When those cells were activated to divide, they produced apparently normal early embryos. Once implanted into female mice, the embryos survived to adulthood at rates roughly comparable to those produced by in vitro fertilization. The biggest difference appears to be a reduction in their growth rates—adults wound up 20 percent smaller than normal mice. This isn't much of a surprise, given that imprinting is one of the ways that males and females compete for providing the appropriate amount of resources for their offspring.

It's not clear whether these mice are necessarily going to be useful for any specific application, but they tell us two things. One is that there appear to be only two regions of the genome that are imprinted in a way that's essential for the control of embryonic development, which should greatly simplify the study of that topic. The second is that the lack of a male contribution has been proposed to be a key reason for the low success rates of cloning and various forms of somatic cell nuclear transplants. The results indicate that there's nothing magical provided by the male other than the properly imprinted chromosomes.

Nature Biotechnology, 2007. DOI: 10.1038/nbt1331

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